The present invention relates to a technique, for use in a semiconductor laser module provided with an optical isolator including a doubly refracting crystal plate and a Faraday element, and an optical fiber amplifier and an optical transfer system using the semiconductor laser module, to stabilize output power by suppressing an influence of returning light emitted by a semiconductor laser element and reflected by an optical fiber.
An optical transfer system utilizes a large number of semiconductor laser elements such as a semiconductor laser element working as a light source of signal light and a semiconductor laser element working as a pump light source in an optical fiber amplifier. Such a semiconductor laser element is built in a semiconductor laser module connected with a transfer optical fiber and an amplification optical fiber. In this case, a laser beam emitted by the semiconductor laser element built in the semiconductor laser module can be reflected by an incidence portion of the optical fiber, resulting in entering the semiconductor laser element as returning light. As a result, an optical output power of the pump light source can be varied and the optical transfer system can be degraded in its noise characteristic.
Furthermore, when a laser beam emitted by a semiconductor laser element working as the pump light source built in the optical fiber amplifier enters another semiconductor laser element, a current output from a monitor PD (photo detector) of the latter semiconductor laser element can be increased. As a result, the output power of the semiconductor laser element cannot be constantly controlled.
As a countermeasure against these problems, an optical isolator, which prevents returning light reflected by an optical fiber and a laser beam emitted by a semiconductor laser element in a pump light source from entering a semiconductor laser element, is generally built in the semiconductor laser module.
A known optical isolator to be built in a semiconductor laser module comprises, as is described in Japanese Laid-Open Utility Model Publication No. 56-49517 and Japanese Laid-Open Patent Publication No. 1-99018, a pair of doubly refracting crystal plates, such as rutile optical crystal plates (hereinafter referred to as the rutiles), and a Faraday element. FIG. 20 shows the sectional structure of a semiconductor laser module described as a first conventional example provided with the optical isolator including the doubly refracting crystal plates and the Faraday element. As is shown in FIG. 20, a laser mount 2 is provided on a Peltier element 1 disposed in a package not shown, and a semiconductor laser element 3 is held on the laser mount 2. A laser beam (whose optical path is shown with a solid line) emitted from an active layer 4 of the semiconductor laser element 3 in one direction (in the leftward direction in FIG. 20) is collected by a collective lens 5 so as to reach an optical isolator 6, passes through the optical isolator 6 and enters an incidence portion of an optical fiber 7. Then, returning light reflected by the incidence portion of the optical fiber 7 (whose optical path is shown with a broken line) passes through the optical isolator 6 and the collective lens 5 and enters the semiconductor laser element 3. On the other hand, a laser beam emitted by the semiconductor laser element 3 in another direction (in the rightward direction in FIG. 20) is detected for its light intensity by a monitor PD 8, and the thus detected intensity is converted into a current signal to be used for controlling the output power of the semiconductor laser element 3.
The optical isolator 6 includes a first doubly refracting crystal plate 11, a second doubly refracting crystal plate 12, a Faraday element 13 disposed between the first and second doubly refracting crystal plates 11 and 12, and a permanent magnet 14 for applying a magnetic filed to the Faraday element 13. The first and second doubly refracting crystal plates 11 and 12 transmit light having a polarization plane parallel to their optical axes (that is, abnormal light) with refraction, and transmit light having a polarization plane perpendicular to their optical axes (that is, normal light) without refraction. The Faraday element 13 transmits incident light with its polarization plane always rotated by a predetermined angle, for example, by 45 degrees, in one direction. FIG. 21(a) shows the direction of the optical axis (Y-axis positive direction) of the first doubly refracting crystal plate 11, FIG. 21(b) shows the direction and the angle (herein 45 degrees in the clockwise direction) for rotating the polarization plane of incident light by the Faraday element 13, and FIG. 21(c) shows the direction of the optical axis (a direction between the Y-axis positive direction and the X-axis positive direction) of the second doubly refracting crystal plate 12. In a coordinate system used in these drawings, a direction parallel to the surface of the active layer 4 of the semiconductor laser element 3 and perpendicular to a direction of a resonator corresponds to the X-axis and a direction perpendicular to the surface of the active layer 4 corresponds to the Y-axis.
In FIG. 22(a), thick solid arrows indicate polarization directions of output light emitted by the semiconductor laser element 3 in one direction toward the optical fiber 7, and the arrow shown in (i) indicates the polarization direction immediately after it is emitted from the active layer 4 of the semiconductor laser element 3, the arrow shown in (ii) indicates the polarization direction when it passes through the collective lens 5, the arrow shown in (iii) indicates the polarization direction when it passes through the first doubly refracting crystal plate 11, the arrow shown in (iv) indicates the polarization direction when it passes through the Faraday element 13, the arrow shown in (v) indicates the polarization direction when it passes through the second doubly refracting crystal plate 12, and the arrow shown in (vi) indicates the polarization direction when it reaches the incidence portion of the optical fiber 7. In FIG. 22(b), thick solid arrows indicate polarization directions of returning light reflected by the optical fiber 7, and the arrow shown in (6) indicates the polarization direction immediately after it is reflected by the incidence portion of the optical fiber 7, the arrow shown in (5) indicates the polarization direction when it passes through the second doubly refracting crystal plate 12, the arrow shown in (4) indicates the polarization direction when it passes through the Faraday element 13, the arrow shown in (3) indicates the polarization direction when it passes through the first doubly refracting crystal plate 11, the arrow shown in (2) indicates the polarization direction when it passes through the collective lens 5, and the arrow shown in (1) indicates the polarization direction when it returns to the semiconductor laser element 3. Description of thick broken arrows in FIG, 22(b) will be given later.
In FIG. 22(a), the output light emitted by the semiconductor laser element 3 is emitted in a TE mode having merely a polarization plane component parallel to the active layer 4 as is shown in (i), and passes through the collective lens 5 with its polarization direction unchanged so as to reach the optical isolator 6 as is shown in (ii). Since the light entering the optical isolator 6 is the normal light having the polarization direction perpendicular to the optical axis of the first doubly refracting crystal plate 11, it passes through the first doubly refracting crystal plate 11 without refraction as is shown in (iii), and is rotated by the Faraday element 13 in the clockwise direction by 45 degrees as is shown in (iv). Therefore, the light is changed to the normal light having the polarization direction perpendicular to the optical axis of the second doubly refracting crystal plate 12, and hence, it passes through the second doubly refracting crystal plate 12 without refraction as is shown in (v) and enters the incidence portion of the optical fiber 7.
In FIG. 22(b), the light is changed to the returning light by being reflected by the incidence portion of the optical fiber 7 with its polarization direction unchanged as is shown in (6). Since the light is the normal light having the polarization direction perpendicular to the optical axis of the second doubly refracting crystal plate 12, it passes through the second doubly refracting crystal plate 12 without refraction as is shown in (5), and is rotated by the Faraday element 13 in the clockwise direction by 45 degrees as is shown in (4). Therefore, the light is changed to the abnormal light having the polarization direction parallel to the optical axis of the first doubly refracting crystal plate 11, and hence, is refracted by the first doubly refracting crystal plate 11 in the Y-axis positive direction as is shown in (3). Then, the resultant light passes through the collective lens 5 with being symmetrically projected in the vertical direction and the horizontal direction by the collective lens 5 as is shown in (2), and is collected onto the semiconductor laser element 3 in a position moved from the output position of the output light in the Y-axis negative direction as is shown in (1).
In this manner, in using the optical isolator built in the semiconductor laser module of the first conventional example, the light output from the active layer 4 of the semiconductor laser element 3 does not enter the active layer 4 but is collected onto the position moved from the active layer 4 in the Y-axis negative direction. Thus, the optical isolator can exhibit its function as an isolator.
Next, an optical fiber amplifier having a high output power of +24 dBm disclosed in, for example, TECHNICAL REPORT OF IEICE EDM 96-39, CPM 96-62, OPE 96-61, LQE 96-63 will be described as a second conventional example. FIG. 23 shows the structure of the optical fiber amplifier of the second conventional example, and the optical fiber amplifier includes an amplification optical fiber 20 doped with rare earth ions, such as erbium ions, and a first semiconductor laser pump light source 21 and a second semiconductor laser pump light source 22, each including the semiconductor laser module of the first conventional example. First pump light emitted by the first semiconductor laser pump light source 21 enters the amplification optical fiber 20 through a first coupler 23, and second pump light emitted by the second semiconductor laser pump light source 22 enters the amplification optical fiber 20 through a second coupler 24. Thus, the erbium ions included in the amplification optical fiber 20 are pumped. As a result, an optical signal input through an incidence portion 20a of the amplification optical fiber 20 is amplified during it passes through the amplification optical fiber 20, so as to be output from an output portion 20b of the amplification optical fiber 20.
In FIG. 23, a solid arrow shows the state of the second pump light emitted by the second semiconductor laser pump light source 22 to enter the first semiconductor laser pump light source 21, and a broken arrow shows the state of the first pump light emitted by the first semiconductor laser pump light source 21 to enter the second semiconductor laser pump light source 22.
Furthermore, in FIG. 22(b), the thick solid arrows and the thick broken arrows indicate polarization directions of light emitted by a semiconductor laser element of the first or second semiconductor laser pump light source 21 or 22 and entering another semiconductor laser element of the second or first semiconductor laser pump light source 22 or 21. The polarization directions shown with the thick solid arrows have already been described, and the polarization directions shown with the thick broken arrows will now be described. In FIG. 22(b), the light having the polarization direction as is shown in (6) is the abnormal light having the polarization direction parallel to the optical axis of the second doubly refracting crystal plate 12, and hence, it is refracted by the second doubly refracting crystal plate 12 in a direction between the X-axis positive direction and the Y-axis positive direction as is shown in (5). Then, it is rotated by the Faraday element 13 in the clockwise direction by 45 degrees as is shown in (4). As a result, the light is changed to the normal light having the polarization direction perpendicular to the optical axis of the first doubly refracting crystal plate 11, and passes through the first doubly refracting crystal plate 11 without refraction as is shown in (3). Then, the resultant light passes through the collective lens 5 with being symmetrically projected by the collective lens 5 in the vertical direction and the horizontal direction as is shown in (2), and enters the semiconductor laser element 3 in a position moved from the active layer 4 in a direction between the X-axis negative direction and the Y-axis negative direction as is shown in (1).
In a semiconductor laser element made from a GaAs material for emitting a laser beam in a region between visible light and near infrared rays, the aforementioned returning light can be absorbed in the semiconductor laser element because a loss of a semiconductor laser beam is large in an area excluding the active layer.
In a semiconductor laser element made from an InGaAsP material for emitting a laser beam in a longer wavelength band for use in optical signal transfer, however, a loss of a semiconductor laser beam is small in the area excluding the active layer. Therefore, the returning light reflected by the optical fiber and output light from a semiconductor laser element of the pump light source cannot be absorbed in the area excluding the active layer of the semiconductor laser element. Accordingly, the light intensity detected by the monitor PD is increased, and the output power of the semiconductor laser element cannot be constantly controlled.
In this manner, although the optical isolator using the doubly refracting crystal plates such as rutile crystal plates can decrease a cost, the monitored light intensity is affected by the returning light when it is built in a semiconductor laser module. This leads to a first problem that the optical output power of the semiconductor laser element cannot be constantly controlled.
Furthermore, heat generated in the active layer of the semiconductor laser element is transferred through an area between the active layer and the laser mount (namely, a lower area in FIG. 20) of the semiconductor laser element and through the laser mount to be absorbed (cooled) by the Peltier element. At this point, the laser mount has high heat conductivity but the area between the active layer and the laser mount in the semiconductor laser element has low heat conductivity. In accordance with increase of the output power of the semiconductor laser element, a larger amount of heat is generated in the active layer. Therefore, the area between the active layer and the laser mount in the semiconductor laser element is heated, and the active layer can be resultantly heated. This leads to a second problem that the output power of the semiconductor laser element cannot be increased.
Moreover, the optical fiber amplifier of the bidirectional pumping system of the second conventional example is good in the amplifying ability, but the output power of the pump light source is desired to be further increased because of demands for further improvement in the amplifying ability. Therefore, when the output level of signal light is low, pump light emitted by the semiconductor laser element of one of the pump light sources can enter the semiconductor laser element of the other pump light source. When the pump light emitted by one pump light source thus enters the semiconductor laser element of the other pump light source, the light intensity detected by the monitor PD can be increased similarly by the returning light reflected by the optical fiber described above. As a result, it becomes difficult to constantly control the output power of the semiconductor laser element.
Furthermore, when the pump light emitted by one of the pump light sources is reflected by the output face of the semiconductor laser element of the other pump light source, the reflected light is scattered in the package of the module. This also increases the light intensity detected by the monitor PD, resulting in similarly causing the problem that the output power of the semiconductor laser element cannot be constantly controlled.